Pulse width modulation (PWM) has been extensively applied to various electronic devices. For example, PWM controllers are used in switching mode power supplies to modulate duty cycles or switching frequencies of power switches and thereby modulate output voltages.
Recently, due to energy shortages and the rising awareness of environmental protection, the energy saving feature of switching mode power supplies has drawn more and more attention. At the same time, laws and regulations were passed to impose stricter requirements on the power conversion efficiency of switching mode power supplies at light load and in standby mode. When a switching mode power supply is working at light load or in standby mode, the switching loss of its power switches accounts for a significant proportion of the overall power consumption. To increase the light loading and standby power conversion efficiency of switching mode power supplies, some power management integrated circuits (ICs) on the market are designed to lower the switching frequencies of power switches so that switching loss can be significantly reduced. Moreover, switching mode power supplies, though advantageously more compact than conventional linear power supplies, have another problem that electro-magnetic interference (EMI) caused by the switching elements. Jittering frequency technique is typically used to improve EMI problem in existing power management ICs.
Switching mode power supplies have a variety of types. While the feedback loop and PWM loop designs vary from one type to another, the PWM controllers in all such power supplies generate and control their PWM signals according to output feedback signals, which may be either voltages or currents. For instance, the switching mode power supply shown in FIG. 1 has a flyback configuration in which the PWM controller 10 needs the output information provided by an isolated feedback circuit that includes an optical coupler 12 and a shunt regulator 14. The shunt regulator 14 detects the output voltage Vo of the flyback power supply and controls the feedback current Icomp on the pin COMP of the PWM controller 10 accordingly. Based on the feedback current Icomp, a circuit in the PWM controller 10 generates a feedback voltage Vcomp which is proportional to the output voltage Vo. From the feedback voltage Vcomp, the PWM controller 10 can identify whether the flyback power supply is operating at light load or heavy load.
The flyback power supply shown in FIG. 1 provides the output power
                                                        Po              =                            ⁢                                                (                                      1                    /                    2                                    )                                ×                Lp                ×                                                      (                                          X                      ⁢                                                                                          ⁢                      1                      ×                                              Vcs                        /                        Rcs                                                              )                                    2                                ×                fs                ×                η                                                                                                        =                                ⁢                                                      (                                          1                      /                      2                                        )                                    ×                  Lp                  ×                                                            (                                              X                        ⁢                                                                                                  ⁢                        2                        ×                                                  Vcomp                          /                          Rcs                                                                    )                                        2                                    ×                  fs                  ×                  η                                            ,                                                          [                  Eq          ⁢                      -                    ⁢          1                ]            where Lp is the magnetizing inductance of the transformer T1, Rcs is the current sense resistor, Vcs is the voltage across the current sense resistor Rcs, fs is the switching frequency of the power switch M1, η is the conversion efficiency of the transformer T1, and X1 and X2 are constant coefficients.
The switching frequency of a conventional constant frequency switching mode power supply with jittering frequency is not affected by the output feedback signal. Taking the flyback power supply shown in FIG. 1 for example, the equation Eq-1 shows that, if the output power Po is fixed, the feedback voltage Vcomp will vary with the jittered switching frequency fs. Referring to FIG. 2, the waveform 20 represents the switching frequency fs having a fixed jittering frequency range Δfs, and the waveform 22 represents the feedback voltage Vcomp. The feedback voltage Vcomp decreases as the switching frequency fs increases. However, when working at light load or in standby mode, this type of PWM controller cannot reduce the switching frequency fs according to the feedback voltage Vcomp to reduce switching loss.
On the other hand, the switching frequency of a conventional variable frequency switching mode power supply with jittering frequency is adjustable by an output feedback signal; that is to say, the light-load or standby-mode switching frequency can be reduced according to the output feedback signal. Taking the flyback power supply shown in FIG. 1 for example, after entering the light-load (frequency reduction) mode, under a constant output power Po, the variable frequency PWM controller 10 with jittering frequency will lower the switching frequency fs according to the feedback voltage Vcomp. At the same time, however, the feedback voltage Vcomp changes with the jittered switching frequency fs. Thus, a relationship is formed between the feedback voltage Vcomp and the switching frequency fs, and a new stable equilibrium point is eventually reached after back-and-forth adjustments. Referring to FIG. 3, the waveforms of the switching frequency fs and of the feedback voltage Vcomp are changed from the waveforms 30 and 32 to the waveforms 34 and 36, respectively. Because of that, the jittering frequency range of the switching frequency fs is narrowed down from Δfs1 to Δfs2, which nevertheless results in increased EMI during the frequency reduction mode.